1. Field of the Invention
The present invention relates to an anti-skid apparatus for use in an automotive vehicle, and more particularly, to an anto-skid apparatus for installation in a vehicle braking system between a master cylinder and wheel brake cylinders to prevent the vehicle wheels from locking in braking operation.
2. Description of the Prior Art
As one such anti-skid apparatus as described above, Japanese Patent Publication for Opposition in 1984 under Publication No. 49-28307 discloses an anti-skid apparatus wherein an input control valve and an output control valve are disposed in parallel with each other in a primary fluid passage connecting a master cylinder and a wheel brake cylinder, and is disposed a fluid pump for pumping a brake fluid from a reservoir connected to the output control valve and returning the pumped brake fluid via a pump passage to the primary fluid passage at a point thereof between the master cylinder and the input control valve. Further, Japanese Patent Application laid open in 1981 under Publication No. 56-142733 discloses and anti-skid apparatus wherein a solenoid valve selectively placed in one of a forst operating position for communication of the wheel brake cylinder with the master cylinder, and a operating position for communication of the wheel brake cylinder with the reservoir is disposed in the primary fluid passage, in lieu of the input and output control valves, and wherein a check valve is also disposed in a portion of the primary fluid passage between the master cylinder and the point to which the fluid pump is connected through the pump passage. This check valve is provided to prevent a so-called "kick-back" phenomenon of a brake pedal which causes unpleasant shocks on the brake pedal. The "kick-back" phenomenon is conventionally encountered due to a return flow of the brake fluid from the fluid pump to the primary fluid passage, and to the master cylinder.
In the anti-skid apparatus disclosed in the Publication No. 56-142733, the check valve is opened if the pressure on the side of the fluid pump becomes lower than the pressure on the side of the master cylinder, while the check valve is in its closed position to block a flow of the brake fluid from the fluid pump toward the master cylinder. To prevent this phenomenon, in U.S. Pat. No. 4,618,189, it has been proposed to provide an anti-skid pressure control means further comprising a check valve disposed in a portion of the primary fluid passage between the master cylinder and the shut-off valve (i.e.,the aforementioned check valve) and adjacent to an accumulator, and a valve-control mechanism disposed between the check valve newly provided and the accumulator for controlling this newly provided check valve. This newly provided check valve, which is normally held in its open position, is provided to block a flow of the brake fluid from the master cylinder toward the shut-off valve. Accordingly, the shut-off valve will not be opened even if the hydraulic pressure in the master cylinder becomes higher than that in the pump passage. In the above-described anti-skid pressure control means, however, the valve-control mechanism and the shut-off valve are quite complicated because they are arranged to be operated in response to the hydraulic pressure of the pumped brake fluid. In addition, although the pressurized brake fluid from the fluid pump is stored in the accumulator if the hydraulic pressure in the master cylinder is higher than that in the pump passage, the brake fluid is pressurized by the fluid pump irrespective of a variation of the hydraulic pressure in the wheel brake cylinder to which the pressurized brake fluid is applied. Thus, the fluid pump is often activated to pressurize the brake fluid at a higher level than a necessary level, which results in not only a loss of energy but also a need for large capacity of the fluid pump. Accordingly, the anti-skid pressure control means is still expensive.